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Abstract

Cannabis sativa L. is a drug producing crop that is illegally cultivated
in South Africa. The South African Police Service (SAPS) use aerial
spotters on low flying fixed wing aircrafts to identify cannabis from
other land cover. Cannabis is usually intercropped with maize to
conceal it from law enforcement officers. Therefore the use of remote
sensing in identifying and monitoring cannabis when intercropped with
maize and other crops is imperative.
This study aimed to investigate the potential of hyper spectral indices to
discriminate cannabis from maize under different cropping methods,
namely, monocropped and intercropped. Cannabis and maize were
grown in a greenhouse. The spectral signatures were measured in a
dark room environment. Green pigments (chlorophyll and carotenoid)
from the treatments were also measured. These pigments were then
compared with their respective indices. Photosynthetic reflective index
(PRI) and Carotenoid Reflective Index (CRI) were two of the indices
used to discriminate cannabis from maize using carotenoid content
while the Red Edge Position (REP) and the narrow band Normalized
Difference Vegetation Index (NDVI) used chlorophyll content and
morphological differences respectively to discriminate the two plant
species.
CRI and NDVI proved to be capable of identifying cannabis under the
two cropping conditions. NDVI showed a 25% spectral over lap for the
monocropped treatments and 60% over lap for the intercropped
treatments. CRI displayed 18% and 58% over lap for the monocropped
and intercropped treatments, respectively. As a result CRI emerged as
the most suitable index for discriminating cannabis from maize. With
proper calibration of airborne or space borne imagery, the study offers
potential to detect cannabis using remote sensing technology.